Jim didn't want Katie to dial his extension.

Season 2. Katie calls the office, and gets Pam, and Pam - annoyed that she has to talk to Jim's girlfriend - says "you can just give her your extension." It's a clever moment that shows how Pam definitely has feelings for Jim.

But consider this:

When Jim gave Katie his number, he would have known that she'd get Pam when she called in. If he wanted to spare Pam's feelings, he could have given Katie his extension. Jim isn't dumb. Maybe he didn't give Katie the extension because he wanted Pam to chat with Katie. Maybe it was a "I can't wait around forever" nudge.

Just a fan theory. Some may have already had it. Have a nice Thursday (Dwight thinks that it's Friday)